The present disclosure relates to wireless communications, and more particularly to multiple-input multiple-output communication systems.
Demand for wireless communication and data processing systems is on the rise. Inherent in most communication channels are errors introduced when transferring frames, packets or cells containing data. Such errors are often caused by electrical interference or thermal noise.
Data is often encoded at the transmitter, in a controlled manner, to include redundancy. The redundancy is subsequently used by the receiver to overcome the noise and interference introduced in the data while being transmitted through the channel. For example, the transmitter might encode k bits with n bits where n is greater than k, according to some coding scheme. The amount of redundancy introduced by the encoding of the data is determined by the ratio n/k, the inverse of which is referred to as the code rate. Codewords representing the n-bit sequences are generated by an encoder and delivered to a modulator that interfaces with the communication channel. The modulator maps each received sequence into a symbol. In M-ary signaling, the modulator maps each n-bit sequence into one of M=2n symbols. Data in other than binary form may be encoded, but typically data is represented by a binary digit sequence.
At the receiving end of a transmission channel, the coded symbols must be decoded. The Viterbi algorithm is an efficient maximum-likelihood sequence detection method for decoding convolutional and trellis coded symbols transmitted over AWGN channels.
In accordance with the Viterbi algorithm, for each received signal, a distance between that signal at time tiand all the trellis paths entering each state at time ti is calculated. In the Viterbi algorithm, the minimum Euclidean distance is selected as the optimum branch metric for decoding convolutional and trellis sequences transmitted in AWGN channels.
One type of wireless communication system is a multiple input multiple output (MIMO) system. In a MIMO system, the transmitter includes multiple transmit antennas and the receiver includes multiple receive antennas. The transmitter splits the data to be transmitted into a number of streams (typically bit streams) and transmits the streams via the multiple transmit antennas. The receiver receives the transmitted streams via the multiple receive antennas.
MIMO communication systems can benefit from knowing the channel state information (CSI) at the transmitter side. For example, in time-division duplex (TDD) systems both forward link (FL) transmissions and reverse link (RL) transmissions make use of the same spectrum and therefore the substantially the same physical channel. The equivalence between FL and RL channels, also referred to as channel reciprocity, enables the use of the CSI acquired during the reception phase, e.g. channel estimation at the base transceiver station (BTS) during the RL and/or at the mobile station (MS) during the RL for the subsequent transmission phase, e.g., FL at BTS and/or RL at MS.
One well known technique that uses CSI to improve transmission performance is commonly known as the eigenbeamforming. In accordance with the eigenbeamforming technique, a number of signals are transmitted along the directions of the principal components of the MIMO channel. If the CSI is assumed not to contain any mismatches, the eigenbeamforming maps a MIMO channel into a set of equivalent single-input-single-output (SISO) communication channels. These channels are subsequently used to transmit either multiple coded streams (tuned to the respective principal components) or a single coded stream spread over the multiple SISO channels.
Another well known technique that uses the CSI to improve transmission performance is commonly known as layering. In accordance with the layering technique, multiple coded streams are transmitted over different layers. The receiver successively decodes the streams associated with each layer and removes the contributions of the decoded layer from the received signal, thereby reducing interference for the following layers. The layering technique provides for achieving a high rate in the absence of accurate CSI at the transmitter. Hence, the layering technique with multiple coded streams is more suited in situations where the CSI may vary. Such a situation occurs in mobile cellular TDD systems where the CSI acquired due to the FL/RL reciprocity degrades as the channel variation rate grows.